Applicants claim priority under 35 U.S.C. xc2xa7119 of German Application Nos. 101 10 154.6, 101 45 202.0 and 102 06 385.0 filed Mar. 2, 2001, Sep. 13, 2001 and Feb. 15, 2002, respectively.
This invention relates to a diversity receiving system for digitally modulated terrestrial and/or satellite radio signals in the frequency range above 1 GHz for motor vehicles, with an antenna arrangement whose received signal is supplied to a radio receiver. In particular the OFDM method (orthogonal frequency division multiplex) and the MPSK method (phase shift keying with M phase conditions) are applied for the radio transmission of digital signals in the frequency range above 1 GHz. For the downlink of a satellite radio connection, the QPSK modulation (4 phase conditions) is frequently selected, and for terrestrial communication, the OFDM modulation is selected because the latter has a lower sensitivity with respect to traveling time differences between signals superimposing each other because of multi-way propagation. This also applies to the satellite radio system SDARS, which is designed for the area-covering mobile radio reception in the USA, and for terrestrial radio broadcasts which take place in heavily populated regions or overcrowded areas in addition to the radiation from 2 satellites.
The transmission disturbances occurring due to the multi-way propagation in connection with mobile reception have been successfully drastically reduced over the years with multi-antenna systems for transmitting analog transmitted signals such as, for example in connection with FM radio transmission. These systems are known from German patent P 3618452.7; P 4034548.3; and P 3926336.3. Because of the structure of OFDM or MPSK signals, these systems cannot be used with digital modulation. The present invention is based on European patent EP 1041736 A2. This patent describes and shows in FIGS. 1 and 2a diversity receivers for OFDM signals as prior art, in which, in connection with the transmission of the OFDM burst, preamble signals are transmitted outside of the time slot provided for the data transmission for the synchronization, channel estimation and antenna selection according to a level criterion. This method has the drawback that the time slot for the preamble signal denoted in FIG. 2 by reference numeral 11 has to be provided for, in the antenna selection in the transmitted burst signal. The most favorable antenna signal can be obtained exclusively depending on the occurrence of the burst signal, and not adapted to the necessity of updating, resulting from the driving movement in the multi-way scenario. This is of significance especially at carrier frequencies above 1 GHz. If, deeper level fading events are to be avoided, about ten updating events can be provided over a driving distance amounting to one half wavelength at 1 GHz. The updating then has to be repeated at a speed of 150 km/h at time intervals of 350 xcexcs. For COFDM signals according to the DAB method (digital audio broadcasting) on the L-band (1.5 GHz), this would mean, in the reverse case, of having the unacceptable requirement that the speed of the vehicle be limited to 0.5 km/h.
Furthermore, defining a separate time slot for a preamble signal for the antenna election would lead to a reduction of the effective rate of transmittable data. The invention described in the EP document 1041736 A2 cited above, does not exclusively evaluate the signal level in view of the antenna selection, but provides, as a selection criteria for the antenna selected in connection with the subsequent data identification, additional signal errors that can be derived from a defined, known burst signal, such as, for example traveling time and phase effects. However, in this case, the updating of the antenna selection only takes place in response to the transmitted burst signal, and consequently at large time intervals.
Therefore, the present invention provides a diversity reception system, in which the received signal is updated in response to the selection of a more favorable received signal with the lowest possible bit error rate, at adequately short time intervals. Moreover, when the vehicle is driving at a high speed, and the data flow is being identified at the same time with a low cost cable link of the radio connection, the bit error rate is kept as low as possible, assuming that a user-friendly antenna is employed from the point of view of current motor vehicle engineering.
The advantage of using the diversity method of the invention is that with respect to the transmitted signals, no measures have to be implemented for using the diversity receiving system on the receiving side. Because of this compatibility, the method can also be used as an addition to a radio system designed for reception with an individual antenna. A further advantage is that the diversity function can also be achieved with only minimal additional expenditure in a receiver intended for reception with a single antenna, because only the intermediate frequency signal, and the symbol cycle signal, which are both available, are required as the minimum requirement that the receiver needs to satisfy. If these two signals are made available on the receiver, the receiving system can be supplemented to obtain an efficient diversity reception system, external of the receiver, by selecting suitable diversity antennas and by a diversity component.
Antennas for receiving systems for the reception of satellite radio signals in motor vehicles are known from German patent DE 40 08 505.8. These antennas are designed as crossed horizontal dipoles, with dipole halves consisting of linear conductor components inclined downwards in the form of a xe2x80x9cVxe2x80x9d. These dipole halves are mechanically fixed in relation to each other at an angle of 90 degrees, and attached to a linear, vertical conductor that is secured on the upper end of a horizontally oriented, conductive base surface. To generate the circular polarization usually required in satellite communications, the two horizontal dipoles inclined downwards in the form of a xe2x80x9cVxe2x80x9d, are electrically wired together via a 90-degree phase network. To receive terrestrially emitted, vertically polarized signals, vertical monopole antennas are provided.
For satellite antennas, an antenna gain of a constant, for example 2 dBi and 3 dBi is strictly required for circular polarization in the elevation angle range of, for example between 25xc2x0 and, respectively 30 degrees, and 60xc2x0 and, respectively, 90 degrees depending on the satellite communication system. This requirement has to be satisfied for an antenna that is built up in the center of a plane, conductive base board. With antennas of this type of construction, the antenna gain required in the range of the zenith angle can be generally obtained without problems. As opposed thereto, the antenna gain required in the range of low elevation angles of from 20 to 30 degrees can be realized only with difficulty and can in no case be realized with a very small structural height of the antennas as required for mobile applications. Specifically, it is also impossible for physical reasons to exceed the 3 dBi-values within the entire three-dimensional angle range and to thus realize an increased quality of the signal.
Antennas bent from linear conductors can be used to satisfy the gain requirements both in the angle range of low elevation, and with steep radiation. The form of antenna frequently used at the present time is the quadrifilar helical antenna according to KILGUS (IEEE transactions on Antennas and propagation, 1976, pages 238 to 241). These antennas often have a length of several wavelengths, and are not known in the form of flat antennas with a low structural height. Also, with an antenna with a low structural height as specified in EP 0 952 625 A2, it is not possible to satisfy the gain values specified above in the low elevation angle range.
In the SAE Technical Paper 2001-01-128 with the title xe2x80x9cXM satellite Radio Technology Fundamentalsxe2x80x9d by Stellios J. PATSIOKAS, a helical antenna for the additional reception of terrestrially transmitted signals is combined for that reason with a monopole antenna, resulting in a large size construction of the combined antenna that is not suitable for use on motor vehicles.
A further problem that exists, in addition to the problem of the structural height, arises in conjunction with these antennas from the fact that because of the build-up required in automobile building on the outer surface of the motor vehicle, in conjunction with the impossibility of placing the antenna in the center of the roof for motor vehicle engineering reasons, or because of the frequently raised demand for integrating the antenna in the shape or form of the vehicle, the direction diagram formed in an idealized manner with the prescribed build-up on the surface is very highly deformed if it is attached to the vehicle, and has impermissible intake problems, as a rule. The ranges with low elevation of the radio waves are also frequently affected. The properties of circular polarization of the antenna may be completely lost in this angle range as well. These influences result from the deflections and reflections of the incident waves, which, in the frequency range above 1 to 3 GHz, are often caused on the edges of the vehicle and by the discontinuities of the body of the vehicle such as, for example the roof edge on the rear window, as well as by shading of the wave incidence by parts of the vehicle.
In addition, the received signal evaluated with the directional diagram changes strongly because of reflected waves superimposed on each other due to the movement of the vehicle, which may cause signal cancellations. All of these effects cannot be avoided with an antenna according to the specifications with the help of the build-up of the antenna on a board, and mounted on the vehicle. The impermissibly high bit error rates that result may lead to break-off of the radio connection. By selecting the suitable individual diversity antennas A1, A2, A3, etc. in the antenna system 1 in the diversity reception system as defined by the invention, it is possible to advantageously reduce these effects to a high degree.
Therefore, an important advantage of a diversity reception system as defined by the invention, is the fact that the demand for a single vehicle antenna with a directional diagram with the required circular polarization, such as with an idealized build-up on the prescribed or specified board surface, which cannot be satisfied in practical life, does not have to be separately satisfied for an individual directional diagram of the reception system in the entire three-dimensional angle range, either for the signals transmitted by the satellite or for the terrestrially transmitted signals. By separately realizing the sectoral directional diagrams, which are independent of one another, it is possible, for example to make available a single directional diagram at the selected points in time. This directional diagram has an adequate antenna gain in the required three-dimensional direction, including the small elevation angles that can be otherwise covered only with much difficulty. However, filling up the level fading events of 10 to 20 dB, which are substantial to some extent, over the driving distance, particularly in the area of partial shading, or shading of the directly incident received signals with the help of the diversity reception system as defined by the invention, is distinctly more effective in view of achieving low bit error rates, than meticulously adhering to a prescribed directional diagram on a circular board.
Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings which disclose at least one embodiment of the present invention. it should he understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention.